1. Field of the Invention
The present invention relates to a microlens, in particular, a solid state imaging device including a microlens on the top of its photoreceiving portion where photoelectric conversion is performed, and to a production method thereof.
2. Related Background Art
A microlens is being used in various optical systems. The optical systems include, for example, a solid state imaging device. The solid state imaging device is now in the process of development toward miniaturization of its integrated circuit chip and arrangement of a larger number of pixels on the same so as to meet demand for high resolution images and miniaturized photographing systems. In order to accomplish miniaturization of the chip as well as the arrangement of a larger number of pixels on the same, it is imperative to reduce each pixel size, and a highly sensitive photoelectric converter, an improved S/N, and a larger aperture width are being investigated so as to compensate for the decrease in an electrical output signal accompanying the reduction of the photoreceiving area.
A microlens has been devised in order to allow the aperture width of each pixel in an imaging device to be larger, and the aperture rate of the imaging device can be raised substantially by condensing the light entering a given pixel efficiently into the photoreceiving portion with the microlens. One microlens is provided for each pixel on the top of the photoreceiving portion of the imaging device in such a manner as to correspond to each photoreceiving portion (refer to European Patent Publication No. 948055).
Generally, a photolithographic process is used as the process for forming a microlens. The production process is as follows: first, the top of an imaging device, which includes a photoreceiving portion, is planarized with a transparent resin; second, the photosensitive resin, which is to be a microlens at a final stage, is shaped into islands by photolithography in such a manner that the islands correspond to respective pixels; third, the island-shaped resin pattern is heated and softened so that the surface tension makes its surface spherical; and lastly, the spherical resin patter is set to be a microlens.
FIG. 24 shows a cross-sectional view of a solid state imaging device of the prior art.
On a semiconductor substrate 11, formed are photoelectric converters 1, vertical CCD resisters 12, channel stops 13 and transfer gate areas 14. The semiconductor substrate 11 includes gate dielectric films 16, transfer electrodes 15, light screen films 17, layer to layer insulation thin films 18 and a cover layer 19 on its surface.
On the cover layer 19, formed is a transparent resin layer 2, and on top of that, a microlens array 3 is formed.
In this solid state imaging device, since the interface of the microlens resin with a high refractive index is exposed, the reflection loss of the incident light is large.
As a measure to improve this point, there is disclosed a method of forming an antireflection film of, for example, aluminum fluoride on the surface of the microlens in Japanese Patent Application Laid-Open No. 10-150179.
After performing a large number of experiments repeatedly, the present inventors found that the solid state imaging devices of the prior art potentially have the problems to be resolved as described below.
Specifically, in the solid state imaging devices of the prior art, since the surfaces of their microlenses are formed of a highly insulating polymer etc., these surfaces are easy charged. Therefore, dust particles are likely to adhere thereto. Moreover, since the topmost surface of the solid state imaging devices is rough due to the presence of microlenses, removing dust particles adhering thereon is difficult; and, if the pixels are allowed to be even finer, removing dust particles becomes more difficult. These problems are not specific only to the solid state imaging devices.
Accordingly, one of the objects of the present invention is to provide a microlens and a solid state imaging device, both of which can be prevented from being charged and thereby the adhesion of dust particles thereon can be inhibited, and a production process thereof.
Another object of the present invention is to provide a microlens characterized by a conductive surface, a solid state imaging device including the above microlens, and a production process thereof.
Still another object of the present invention is to provide a production process of a microlens or a solid state imaging device characterized by including:
a step of forming a resin pattern on the top of a substrate;
a step of forming the above resin pattern;
a step of irradiating the above shaped resin pattern with ultraviolet rays and heat-treating the same; and
a step of changing the surface of the above resin pattern into a conductive surface.
According to an aspect of the present invention, there is provided a microlens, wherein a surface of the microlens is a conductive surface.
According to another aspect of the present invention, there is provided a method for producing a microlens, comprising the steps of:
forming a resin pattern on or above a substrate;
forming the resin pattern;
irradiating the formed resin pattern with ultraviolet while subjecting a heat treatment to the formed resin pattern; and then
altering a surface of the resin pattern into a conductive surface.
According to another aspect of the present invention, there is provided a microlens, a surface of which is coated with a light transmitting conductive film and a light transmitting low-refractive-index film with a refractive index lower than that of a component of the microlens.
According to another aspect of the present invention, there is provided a method for producing a microlens, comprising the steps of:
forming a resin pattern on or above a substrate;
forming the resin pattern;
irradiating the formed resin pattern with ultraviolet while subjecting a heat treatment to the formed resin pattern; and then
forming a light transmitting conductive film and a low-refractive-index film with a refractive index lower than that of a component of the microlens.
According to another aspect of the present invention, there is provided a solid state imaging device comprising a photoreceiving portion for performing a photoelectric conversion and a microlens provided on the photoreceiving portion, wherein a surface of the microlens is a light transmitting conductive surface.
According to another aspect of the present invention, there is provided a method for producing the above-described solid state imaging device, comprising the steps of:
forming a resin pattern, which is to be the microlens, on or above the photosensitive portion;
forming the resin pattern;
irradiating the formed resin pattern with ultraviolet while subjecting a heat treatment to the formed resin pattern; and then
altering a surface of the resin pattern into a conductive surface.
According to another aspect of the present invention, there is provided a solid state imaging device comprising a photoreceiving portion for performing a photoelectric conversion and a microlens provided on the photoreceiving portion, wherein a surface of the microlens is coated with a light transmitting conductive film and a low-refractive-index film with a refractive index lower than that of a component of the microlens.
According to another aspect of the present invention, there is provided a method for producing the just above-described solid state imaging device, comprising the steps of:
forming a resin pattern to be the microlens on or above the photoreceiving portion;
forming the resin pattern;
irradiating the formed resin pattern with ultraviolet while subjecting a heat treatment to the formed resin pattern; and then
forming the light transmitting conductive film and the low-refractive-index film.